A plate floor heat exchanger is particularly applicable with advantage wherever the heat-transfer coefficient of the medium flowing in the channel is much greater than that of the medium flowing among the plate floors. Such conditions exist, in general, in air coolers, air cooled condensors, air heaters, air radiators and air conditioning plants.
In known devices for such purposes one of the media taking part in the heat exchange flows in the closed profile channel of an optional cross section, while the other medium flows among the plate floors. The space between the plate floors is maintained by means of spacers which can be separate spacer elements (spacer rings) or flanges that are formed on the plate floor.
A characteristic of such devices is that the spacer elements, and channels, respectively, during operation generate significant resistance to flow of the medium along the state floors. In the "wind shadow" of channels and spacer elements, respectively, and thus along the sides thereof opposite the direction from which the flowing medium between the plate floors encounters them a so called "dead space" is formed within which heat transfer is brought about not by means of flow but practically only through convection.
As a consequence the surfaces defining the dead space practically do not take part in heat transfer. Moreover the turbulence disengagements developing in the dead space increase to a significant extent the resistance of medium and therefore, the flow of the medium in the space between the plate floors requires a rather greater input. If the spacer element is formed by the flanging-out of the plate floor, heat transfer will be impaired also by the thinning of the plate floor material as a consequence of the flanging-out.
To avoid or limit these disadvantages, different proposals have been made. The essence of these proposals is in the interest of reducing the resistance of the medium and the dead space by forming the channels of tubes having oval or elliptical cross section. The tubes are elongated in the flow direction of medium flowing along the plate floors. Such a solution is described in German Patent No. 2,123,723 other publications as well see: Transactions of ASME, Series "B", May 1966.
A characteristic of the oval tube construction and similar solutions is that, though the dead spaces are reduced in size they are not eliminated, and thus the flow properties of plate floor heat-exchangers shaped this way are more favorable, they can be improved still further.
In the use of oval or elliptical tubes it can be assured only with difficulty that the metal connection guaranteeing good heat conduction between the channel and plate floors will continue during the whole period of operation. Should a tube having such a cross section be placed under working or test pressure, the tube under the effect of pressure tends to assume circular cross-section. The repeated change of shape may loosen the metal connection between the tube and the plate floor, thus impairing heat transfer.
A tube having an oval or elliptical cross-section also has less strength and its fabrication is more complex a therefore more expensive.